This invention relates generally to meltblowing dies and specifically to a meltblowing die with a replaceable die tip. In another aspect, the invention relates to a method of replacing the die tip of a meltblowing die.
Meltblowing is a process for manufacturing nonwoven products by extruding a molten thermoplastic resin through a plurality of small orifices to form fine fibers while blowing converging sheets of air onto each side of the orifices to attenuate and draw down the fibers. The extruded fibers are blown onto a moving collector surface where they entangle in a random way to produce a nonwoven fabric or web. The newly formed web is directly withdrawn from the collector for winding and the overall process is carried out continuously. The webs may be further processed into a number of commercially important products such as filters, fabrics, and battery separators to name a few. Some meltblowing dies are disclosed in U.S. Pat. Nos. 3,978,185; 4,818,463; and 4,986,743.
A key component of a meltblowing die is the die tip. The die tip is an elongate member having an outwardly protruding nose piece of triangular cross section terminating in an apex with a row of side-by-side orifices drilled through the apex. A polymer melt is forced through the orifices for extruding the polymer and discharged as side-by-side molten or semimolten fibers. The die tip is generally formed of high quality steel in a separate machining process and fastened to the face of the die body using bolts. Precise machining is required to achieve uniform polymer flow over the length of the orifice row. The polymer is forced through the die tip orifices by applying large pressure on the molten polymer inside the tip. The internal pressure induces tensile stress in the die tip which tends to concentrate near the apex of the tip and may cause the tip to fail in tension. U.S. Pat. No. 4,986,743 teaches the use of die tip mounted on the die body with induced compression in the apex region to counterbalance the tensile stress and is incorporated herein by reference.
Other important components of the meltblowing die are elongate plates referred to as air knives which, in combination with the die tip nose piece, form converging air flows to attenuate and draw down the extruded fibers to microsized diameters. The air knives are generally thick elongate plates which have longitudinal edge tapered to form a knife edge. Two air knives are typically used and are fastened to the face of the die body (using bolts) on opposite sides of the triangular die tip nose piece. The tapered edges of the air knives are aligned with the confronting tapered surfaces of the nose piece and spaced slightly therefrom to form two flow channels which converge at the apex of the nose piece. The spatial relationship between the air knives and the die tip is defined in the art by parameters known as air-gap and set-back. The air-gap and set-back determine the geometry of the converging air flow passages, and thereby influence the air flow properties and the degree of fiber-air interaction. Research has shown that controlling the air flow properties in relation to the polymer flow properties is important for achieving the desired degree of fiber attenuation and final fiber diameter. Research has further shown that the fiber diameter strongly influences the properties of She web such as filtration efficiency. In some die designs, the air-gap and set-back are adjustable for achieving the desired air flow properties. Meltblowing polymers with different compositions may require different air-gap and set-back.
A common problem in meltblowing occurs when the die tip plugs or structurally fails requiring that the die tip be removed for cleaning or replacement. In a conventional meltblowing die, the die tip and air knives are separate components which are individually bolted to the face of the die (e.g., those shown in U.S. Pat. Nos. 4,818,463 and 4,986,743). Removal of the die tip generally requires first that each air knife be detached from the die body to gain access to the die tip. Large meltblowing dies are normally vertically oriented and discharge downwardly onto a moving collector surface such as a conveyor screen or rotating drum. For removing each air knife, the air knife bolts are removed, the air knife lowered From the die face and withdrawn from the workspace. The die tip bolts are then removed and the tip is lowered and withdrawn. For large dies this procedure may be hazardous due to the large weight of the air knives and die tip. For dies employing a large conveyor-type collector, it may not be feasible to move either the die or collector to facilitate replacement of a fouled or damaged die tip, and this further complicates the procedure due to the restricted space between the die and collector.
Similar difficulties to those described above are encountered in the reverse installation procedure, where yet another serious problem is the adjustment of the air-gap and set-back after the die tip and air knives have been reattached.
In addition to safety problems, the die tip may be damaged as it is maneuvered into or out of the die. In typical operation, the die tip and air knives may be at temperatures above four hundred degrees Fahrenheit and the meltblowing line must be shut down for an extended period to allow for cooling before repairs can be made or die tip replacement effected.